A semiautomatic vehicle in which a clutch provided in the torque transmission path from the engine to the wheels is operated by an actuator and, in addition to the operation of the clutch, a transmission disposed downstream of the clutch is operated by an actuator is conventionally known. See, for example, Japanese Patent Application Publication No. 2001-173685 (JP-A-2001-173685).
Some straddle-type vehicles, such as motorcycles, have a friction clutch including a driving-side friction member (e.g., a friction disc) and a driven-side friction member (e.g., a clutch disc), and a constant-mesh transmission including a plurality of gears which are relatively movable axially. In the constant-mesh transmission, dog clutches are formed on the gears and the plurality of gears are selectively moved at the time of gear change and the combination of gears that are engaged with each other by their dog clutches is changed to change the gears used to transmit torque.
In the semiautomatic vehicle having such transmission and clutch, when the rider operates a shift button or shift pedal to command a gear change, the actuator first disengages the clutch to interrupt the transmission of torque to the transmission and then displacement of the gears is started. After the displacement of the gears is completed and the gears are engaged in a combination in accordance with the gear change command, the actuator starts engagement of the clutch, that is, a control operation to shift the clutch from the disengaged state to the engaged state. In this control operation, the clutch is typically controlled to shift to the engaged state gradually, and transmission of torque by the clutch and the transmission starts when pressure is generated between a driving-side friction member and a driven-side friction member.
In the above-described conventional control operation, however, since the clutch is temporarily shifted to a disengaged state, it takes time until the transmission of torque to the wheel starts after the start of engagement of the clutch, resulting in insufficient acceleration performance at the time of gear change.
In this respect, a control operation in which the clutch is engaged before the displacement of the gears is completed, that is, before two gears are engaged with each other by their dog clutches and a control operation in which the gears are displaced and engaged with the clutch kept engaged are contemplated. According to such control operations, torque is transmitted to the wheels via the clutch and the transmission upon engagement of the two gears by their dog clutches. For example, the engine output is reduced to reduce the torque to the transmission with the clutch kept engaged when a gear change command is given, and then displacement of the gears is started. Then, the gears are engaged in a new combination with the clutch still engaged. According to such a control operation, however, the acceleration performance at the time of gear change can be improved but a shock may occur and the ride quality of the vehicle at the time of gear change may be impaired because excessive torque is transmitted to the wheel when the gears are engaged by their dog clutches.